This invention relates to sintered bodies of zirconia for oxygen concentration sensors exhibiting an electromotive force in accordance with the difference in the concentration of oxygen between a gas tested and a reference gas, and more particularly to a sintered body of zirconia for an oxygen concentration sensor suitable for use in detecting the concentration of oxygen in waste gas of internal combustion engines.
This type of sintered body of zirconia has hitherto been produced by subjecting a divalent metallic oxide, such as calcium oxide, etc., or a trivalent metallic oxide, such as yttrium oxide (yttria), ytterbium oxide, etc., and zirconium oxide (zirconia) to heat treatment at high temperatures to cause them to form a solid solution, in order to produce oxygen ion conductivity. In this process, the amount of the divalent or trivalent metallic oxide added to zirconium oxide is selected such that the oxygen ion conductivity is maximized and the sintered body has a cubic crystal structure.
The results of experiments conducted by the inventors of the present invention show that although the sintered body of zirconia having a stabilized cubic crystal structure has a relatively high coefficient of thermal expansion and is not readily damaged by thermal impact when exposed to a gas atmosphere to be tested in which changes in temperature occur gradually, this sintered body of zirconia tends to produce excessively high thermal stress in its interior until it is finally damaged, when exposed to a gas atmosphere to be tested in which sudden changes in temperature occur as is the case with a waste gas of an internal combustion engine. In other words, it has been revealed that the sintered body of zirconia having a stabilized cubic crystal structure is low in thermal impact strength.
The inventors have conducted experiments in an effort to increase the thermal impact strength of sintered bodies of zirconia by solving this problem. As the result of the experiments, it has been ascertained that it is possible to increase the thermal impact strength of sintered zirconia of the stabilized cubic crystal structure by causing sintered zirconia of a monoclinic crystal structure to be present in the first-mentioned sintered zirconia in mingling relation in a sintered composite. In this context, the term "monoclinic" encompasses the monoclinic crystal system or a rhombic crystal system. When exposed to an atmosphere in which temperature rises, a sintered body of zirconia of the monoclinic crystal structure is altered through a tetragonal system into a cubic crystal structure, with an attendant change in volume. Therefore, if this sintered body of zirconia is used with an oxygen concentration sensor, it would be broken to pieces. This non-stabilized sintered body of zirconia of the monoclinic crystal structure has a low coefficient of thermal expansion, so that if it is caused to be present in a sintered body of zirconia of the cubic crystal structure in mingling relation, it would be possible to increase thermal impact strength of the sintered composite.
When exposed to a high temperature of about 800.degree. C. which is prevailing in an waste gas of an internal combustion engine of an automotive vehicle, for example, the monoclinic crystal structure changes into a tetragonal crystal structure, which changes back into the monoclinic crystal structural when rapidly cooled. When this phenomenon occurs, a change occurs in the volume of the monoclinic crystal structural portions or gaps are formed in the grain boundary of the monoclinic crystal structural portions. It is believed that although cracks are formed in the cubic crystal structural portions when the sintered body is rapidly cooled, the gaps formed in the grain boundary of the monoclinic crystal structural portions would inhibit the occurrence of the bridging between the cracks formed in discrete cubic crystal structural portions, if the sintered body of zirconia of the monoclinic crystal structure were present in mingling relation to the sintered body of zirconia of the cubic crystal structure with the result that the cracks would be prevented from increasing their size. It is also believed that the monoclinic crystal structural portions would not readily develop cracks because of their very low coefficient of thermal expansion. It would seem that under these circumstances the sintered body of zirconia has its thermal impact strength increased and does not readily develop cracks. It is known that when a partially stabilized sintered body of zirconia including a monoclinic crystal system and a cubic crystal system existing in mingling relation is cooled from a high temperature level to a low temperature level, the ratio of monoclinic crystal structure increases. However, it has recently been ascertained as a result of research that portions of the cubic crystal structure remain the same phase as that in high temperature state and do not change into the monoclinic phase, when cooled. Even if allowed to stand at room temperature, the majority of the partially stabilized sintered bodies of zirconia do not show changes in the cubic phase and the monoclinic phase so far as X-ray diffraction is concerned. However, experiments conducted repeatedly by the present inventors have revealed that prolonged holding of this type of sintered body of zirconia at the range between 200.degree. and 300.degree. C., particularly at 200.degree. C., or subjecting of this type of sintered body of zirconia to repeated changes in temperature in the aforesaid temperature range causes a change to occur with respect to the X-ray diffraction intensity ratio of the monoclinic crystal structure and the cubic crystal structure, and causes damage to the sintered body, if the change is great.